Observation before and after a chemical experiment

The Water-Candle experiment is an illustrative example. It is a situation where many different effects play together and where it is hard to figure out which ones really matter. My own perspective about this experiment has shifted several times and comments of some who wrote me added valuable insight. Please look also towards the end of this page where some interesting links are added and information like why the great Lavoisier himself replaced the experiment since it was too subtle.

Cover a burning candle with a pitcher so that the candle is in an air-tight room sealed by the water at the ground. After some time, the candle dims and goes out. No air bubbles are seen.

Ozone Monitoring

The water level stays up for many few minutes more. The burning produces water H 2O and carbon dioxide C O 2.

Lavoisier was definitely a great pioneer in this context;

Then the chemistry part would be ruled out and the physics contribution alone can be measured;

General Properties of common Air It appears from all these Experiments, that in each of them phlogiston , the simple inflammable principle, is present;

The section on "What is happening in the experiment" confirms the above picture;

In real experiments, there are differences but they depend on the actual experiment:

This cancels the depletion of the oxygen temporarily and the water level stays down. When the oxygen is depleted, the candle goes out and the air cools. The volume of the air decreases and the water rises.

Classroom Experiments

The temporary temperature change delays the rise of the water. As several readers have pointed out, also the water condensation should be mentioned. While water is initially gas, it condenses and helps to delay the effect. There are two different effects. Both a chemical and a physical reasoning are needed to explain what we can see.

Both physics and chemistry matter. The initial cancellation effect can confuse the observer. Mathematics plays a role when the chemical equations are balanced.

Photos of the experiment Photos: Oliver Knill, September 19, 2006. An exhibit of explanations Many explanations on the web contradict each other September 2006. Actually, the primary hits in a search engine all lead to pages, which difficult.

Here are some pitfalls: Oxygen is replaced by Carbon dioxide. So, there is the same amount of gas added than taken away. Therefore, heat alone most be responsible for the water level change. Source of the Error: A simplified and wrong chemical equation is used, which does not take into account the quantitative changes.

The chemical equation has to be balanced correctly. It is not true that each oxygen molecule is replaced by one carbon dioxide molecule during the burning process; two oxygen molecules result in one carbon dioxide molecule and two water molecules which condense.

Remember oxygen is present in the air as a diatomic molecule. If the experiment were done with the sealing fluid able to support a temperature greater than 212 F and the whole system held above this temperature then the water product of combustion would remain gaseous and the pressure within the vessel would increase as a result of three observation before and after a chemical experiment molecules for every two prior to combustion and the sealing fluid would be pushed out.

Carbon dioxide is absorbed by the water. Thats why the oxygen depletion has an effect. This idea is triggered from the fact that water can be carbonized or that the oceans absorb much of the carbon dioxide in the air. But carbon dioxide is not absorbed so fast by water.

The air would have to go through the water and pressure would need to be applied so that the carbon dioxide is absorbed during the short time span of the experiment. The experiment can be observation before and after a chemical experiment by physics alone. During the heating stage, air escapes. Afterwards, the air volume decreases and pulls the water up. In that case, some air would be lost through the water.

But one can observe that the water level stays up even if everything has gone back to normal temperature say 10 minutes. No bubbles can be seen. It can not be that the oxygen depletion is responsible for the water raising, because the water does not rise immediately.

The water rises only after the candle dims. If gas would be going away, this would lead to a steady rise of the water level, not the rapid rise at the end, when the candle goes out.

It is not "only" the oxygen depletion which matters.

Understanding Observations

There are two effects which matter: These effects cancel each other initially. Since these effect hide each other partially, they are more difficult to detect. What do we learn? An important aspect in pedagogy is to understand "how students learn" and how to produce a classroom atmosphere, in which students learn well. But teaching is complex. Already the material itself can be complex. Getting the facts straight can matter too. It is often the reason for pedagogical failures.

A first step is to get the sources right. How can students learn if the sources are incorrect? This is where linear algebra kicks in. With 20 percent of oxygen in our air, we get about 8 percent of the air volume removed. For paraffin wick used in candles, n is larger than 20. Since CO2 has one carbon atom more than O2, it is heavier. Will this not imply that it takes up more volume? It turns out that only the number N of molecules matters. Like any physical law, this is an idealisation and approximation but it is accurate enough for the experiment in question.

In the candle experiment, the pressure and temperatures at the beginning and the end are essentially the same. A refinement of the law, the van der Waals equation also incorporates the size of the molecules.

Added March 20, 2011 Jonathan Lavian, who writes a research paper for an observation before and after a chemical experiment minor, writes: Many people try to explain the problem with physics alone with a different argument.

Getting the facts right

They argue that less hot air is captured in the cup. In other words, the cup covers a volume of less dense air because the air is heated around the candle. When the air cools after the candle goes out, the pressure decreases almost entirely from less dense air cooling. Regardless, some may argue that the chemical aspect is very minimal because the water level sometimes rises to one third of the volume, but under perfect conditions reaction condition, the reaction chemistry can only account for a maximum ten percent water level rise.

You suggest that the water level rises to one tenth of the height, however it can be much higher if more candles are used. I agree that the chemical reaction can have an effect, but how would you rank the contribution of each? Is one effect minimal or more important? How does the size of the candle or container play a factor?

The ingredients of Ivory soap were analyzed to see whether it really is 99.

Candle experiment done carefully so that initially the water level inside is close to the water level outside.

Note that some classroom experiments, such as those that involve observing chemical behavior, require safety precautions and may need to take place in a laboratory. Over the time the oxygen in the jar is reduced and conditions for burning are changed.

I myself did not make the experiment with several candles but I can imagine that one can boost the physics part like this: I can imagine that this can be substantial and would not be surprised to see the water level rise to 30 percent without contradicting anything said above.

I myself have lighted the candles and then immediately placed it and not waited until the air around it got hot. One could do observation before and after a chemical experiment experiment with an other heat source which does not use any chemical processes.

Then the chemistry part would be ruled out and the physics contribution alone can be measured. To completely rule out preheating, one could light the candle from inside the container. This would have to be done carefully however as gas lighters might contribute additional gas and heat for example. I think it is better to light the candle, place the candle down and then immediately place the pitcher around it. Excessive preheating is excluded like this. The size of the pitcher certainly will have an effect.

If the pitcher is too large, then both the effect of the physics as well as the chemistry will be smaller simply because only part of the room will be affected.

What I liked about the experiment is that with household size objects, one can get directly to a situation where the balance between physics and chemistry is initially equal. The initial cancellation of different effects is what makes the experiment so interesting and puzzling. Here is a local copy retrieved September 26. The section on "What is happening in the experiment" confirms the above picture.

It mentions some additional details like 1 that little Carbon monoxide is produced or 2 that almost all the Oxygen gets used up or 3 that the circular current within the jar makes sure that also Oxygen from above gets used and 4 that before closing the jar over the candle some air might escape. The text also mentions 5 some bubbles which might escape if one is not careful and 6 that water vapor can condensate on the jar. An other subtlety is that 7 through increase in temperature, air becomes unsaturated to accommodate additional water vapors.

To 5 This is a major misunderstanding in many explanations. As the text mentions there are no bubbles if the experiment is done right.